Physics Exam 4 Content

what is direct current

the flow of electric charge only in ONE direction aka a steady state of a constant-voltage circuit

what is alternating current

the flow of electric charge that periodically REVERSES direction. the source varies periodically, specifically sinusoidally

generating AC vs DC

DC: batteries provide DC via a chemical reaction

AC: produced by an alternator; it produces an alternating current

what is the main level between DC and AC

DC: voltage or current stays constant

AC: voltages and current reverse when flow of charge changes periodically

can we have a circuit with a capacitor and resistor?

yes, they are called RC circuits

what is the function of an RC circuit

To control the amount of current flowing through a circuit by limiting the rate at which the voltage across it changes. it can help with filtering signals, timing circuits, and managing voltage surges

in a RC circuit, what happens when the current reaches the capacitor

The charge will build up until it is fully charged, then the capacitor will change as a resistor

for an RC circuit, as t goes to infinity, the charge on the capacitor approaches

Q=Cε .

how do we know if the capacitor is fully charged

Q=C

what is the relationship between time and charging capacitor

it is an exponential relationship

how do capacitors in RC circuits behave at t=0

short circuits, aka when the battery were connected to two resistors R in parallel

how does a capacitor behave when fully charged

like an open switch and will redirect the circuit to only go through one of the resistors (breaks circuit)

how do you discharge a capacitor

by closing the switch in the system

describe the rate at which capacitor discharges

voltage decreases exponentially

how do you find the time constant on the capacitor and what does it represent

R=resistance (ohms) and C=capacitor (F)

it represents how long it takes for the capacitor to get to its new voltage (either as charged or discharged)

charging vs discharging

charging: current flows and charge builds up on the capacitor

discharging: current flows in the opposite direction until charge drops to zero

describe the conditions needed for charging a capacitor

switch is closed and battery is connected to an uncharged RC circuit

describe the conditions needed for discharging a capacitor

a charged capacitor is connected across a resistor, it will discharge (usually no battery)

describe how a charging capacitor will act when t=0 vs when it is fully charged

t=0: will act like a short circuit (draws current)

when fully charged: it will behave is an open circuit and block current (as in not draw any current into it)

how do you calculate charge and current for charging capacitor

what does charge and current look like overtime for a charging capacitor

charge increase (charging) exponentially, and as the capacitor charge increases, it will behave more and more like an open circuit (stopping some current flow)

what does charge and current look like overtime for a discharging capacitor

charge is decreasing because it is discharging and current decreases (started at its maximum as charged and now decreasing)

how do you calculate finding charge and current during discharging

which ones are experiencing repulsion? attraction?

what happens when a bar magnet is broken in half

two new poles appear and each half has a north and south pole

what is wrong with this image

magnetic field lines CANNOT cross.

describe magnetic field lines

they form closed loops, no magnetic charge for them to “begin and end” on. they will point only one direction

describe a magnetic field in terms of north and south pole

magnetic field lines come out of the north pole (emerging) and enter the south pole. inside, they go from south to north

magnetic field lines always form

closed loops

what is unique about Earth’s north and south pole

the north pole is actually the magnetic south pole and the south pole is actually the magnetic north pole. it is because a compass points north bc it is attracted to the south pole. the compass points south towards the actual north pole

what is the formula of the force the charge experience when it is moving through a magnetic field

what does Fb represent here

the magnitude of the magnetic force on a moving charge

what is the magnetic force on a moving charge

what is the unit of 1 gauss in teslas

1 guass = 1 G = 10^-4 T

how do we find the direction magnetic force

right hand rule

index=velocity

middle=magnetic field

thumb=force

what is this direction

into the page

what direction is this vector

pointing out of the page

the right hand rule gives the direction of the force on what kind of charge

positive

you will need to flip your hand to get the negative one

in a magnetic field, the force is what direction to the field

perpendicular

how would you describe the path of the motion of a charged particle? why is this way?

the path of a particle is circular if the magnetic force and velocity are perpendicular to the magnetic field

does the constant magnetic field do work on a particle? will it change the particle’s velocity?

no, so it will not change the particle’s velocity (an electric field can do work on a particle:

what is going on to magnetic and electric forces at a velocity selector

the electric and magnetic forces will be balanced

what is the relationship to velocity, electric, and magnetic forces

at the right speed (velocity), electric and magnetic forces will be balanced

what is a flow meter

where magnetic force separates positive and negative charges to create an electric field/measurable potential difference

how does the flow rate affect force and separation of charges in a flow meter

the faster the flow→the greater the force→more separation the positive and negative charges→stronger potential difference

how do you define the radius of a moving charge in terms of magnetic field and velocity

charged particles moving in a magnetic field resemble what kind of force

centripetal force

if a charged particle’s velocity magnitude is constant, what is its kinetic energy

also constant

what is the relationship between radius of the path of a moving charged particle in a magnetic field and its charge

the larger the charge=smaller the radius

why do we say the magnitude velocity of a charge moving in a constant magnetic field is constant and not just velocity

because the direction is changing but not the actual value. velocity is a vector value

when figuring out which angle to find the magnetic force, do we use the smaller or the larger one

smaller one (sometimes will have to do minus 360)